It is frequently necessary to apply heat-setting treatments to cloth formed of textile fibres to impart dimensional stability that is compatible with the use thereof. This is particularly true for synthetic fibres optionally associated with elastane fibres in knits. Knitted polyamide fibres are therefore treated at temperatures lying between 170 and 200° C. and even above, notably depending upon the elastane content of the knits. An increase in elastane content leads to increasing the heat-set temperature to obtain the required dimensional stability.
Yet this heat setting also has the effect of deteriorating the mechanical properties of yarns, notably if these are polyamide fibres. This mechanical deterioration in itself is detrimental, but factory observations on downgraded batches or laboratory observations systematically correlate this deterioration in mechanical properties with reduced dye affinity of the fibres, and with a problem of level dyeing. It is therefore difficult during subsequent dyeing operations to obtain knits having uniform colour and the desired colour intensity. An increase in the heat-treatment temperature generally leads to increased deterioration of mechanical properties and dye affinity, but the results vary greatly from one batch of fibres to another.
It is also frequently ascertained, in one same faulty batch, that there are different types of dyeing defects (bands, light, very light areas; not fully dyed . . . ). The phenomenon is not only random but can only be ascertained a posteriori, after the dyeing operation. Knits that are not uniformly dyed or too pale in colour have to be corrected with a darker colour or dyed a second time, and are often downgraded which is a causes of major economic loss.
It is frequent in the textile industry to use levelling agents which allow better dispersion of the colouring agent in the dye bath, or which act either through their affinity with the fibre or through their affinity with the dye. These may be non-ionic surfactants for example such as ethoxylated fatty alcohols, ethoxylated or non-ethoxylated fatty acids, ethoxylated fatty amines, alkylphenols and fatty mercaptans, or products containing bisulphate anions and quaternary ammonium compounds. Aside from the fact that some of these compounds prove to be difficult to eliminate, are scarcely biodegradable, even toxic, they do not systematically offset the loss of dye affinity and uneven colouring of polyamide fibres after heat treatment. A few, specific commercial products intended to overcome the loss of dye affinity observed after heat treatment are formulated for example with n methylpyrrolidone, hydroxylamine, polyglycol ether, butyl diglycol. However their efficacy is not systematic and is highly variable from one batch of fibres to another.
There is therefore a need to provide additives for textile auxiliaries that, after heat treatment for dimensional stability, can impart more satisfactory dye affinity and improved dye levelling. There is also a need to obtain this result more systematically, irrespective of the quality of the starting fibre and over the entire heat-treatment temperature range in particular the higher temperatures used in practice which are higher than 180° C. being in the order of 190° C., 195° C., 200° C. and above.